Dont need a bumper. Just a license plate and a light for it. I think there's a requirement for reflective surfaces facing rear, which is why the stock bumper has two little reflectors.
They used to sell trucks without bumpers as a cost savings measure. You could buy the bumper as an add on if you wanted.
Why no bumper at all? I run a Mako rear bumper delete. It gives 2 d ring mounts, and it covers the gaping opening taking the bumper off leaves. Also have the attica 4x4 rear fascia cover.
If you don't have a rear bumper and you do any wheeling, if you come down on something in the back you are going to damage the pinch seams of the body panels.